[U-Boot] [PATCH v3 1/6] spl: add Kconfig option to clear bss early

Tue May 21 21:00:48 UTC 2019

Hi Simon (Glass),

On Sat, May 18, 2019 at 10:08:19AM -0600, Simon Glass wrote:
> Hi Andreas,
> 
> On Mon, 6 May 2019 at 22:49, Andreas Dannenberg <dannenberg at ti.com> wrote:
> >
> > Hi Simon,
> >
> > On Mon, May 06, 2019 at 09:51:56PM -0600, Simon Glass wrote:
> > > Hi Andreas,
> > >
> > > On Fri, 3 May 2019 at 14:25, Andreas Dannenberg <dannenberg at ti.com> wrote:
> > > >
> > > > Simon,
> > > >
> > > > On Sat, Mar 30, 2019 at 09:18:08PM +0100, Simon Goldschmidt wrote:
> > > > > Simon Glass <sjg at chromium.org> schrieb am Sa., 30. März 2019, 21:06:
> > > > >
> > > > > > Hi Simon,
> > > > > >
> > > > > > On Wed, 27 Mar 2019 at 13:40, Simon Goldschmidt
> > > > > > <simon.k.r.goldschmidt at gmail.com> wrote:
> > > > > > >
> > > > > > > This introduces a new Kconfig option SPL_CLEAR_BSS_F. If enabled, it
> > > > > > clears
> > > > > > > the bss before calling board_init_f() instead of clearing it before
> > > > > > calling
> > > > > > > board_init_r().
> > > > > > >
> > > > > > > This also ensures that variables placed in BSS can be shared between
> > > > > > > board_init_f() and board_init_r() in SPL.
> > > > > > >
> > > > > > > Such global variables are used, for example, when loading things from FAT
> > > > > > > before SDRAM is available: the full heap required for FAT uses global
> > > > > > > variables and clearing BSS after board_init_f() would reset the heap
> > > > > > state.
> > > > > > > An example for such a usage is socfpa_arria10 where an FPGA configuration
> > > > > > > is required before SDRAM can be used.
> > > > > > >
> > > > > > > Make the new option depend on ARM for now until more implementations
> > > > > > follow.
> > > > > > >
> > > > > >
> > > > > > I still have objections to this series and I think we should discuss
> > > > > > other ways of solving this problem.
> > > > > >
> > > > > > Does socfgpa have SRAM that could be used before SDRAM is available?
> > > > > > If so, can we not use that for the configuration? What various are
> > > > > > actually in BSS that are needed before board_init_r() is called? Can
> > > > > > they not be in a struct created from malloc()?
> > > > > >
> > > > >
> > > > > The problem is the board needs to load an FPGA configuration from FAT
> > > > > before SDRAM is available. Yes, this is loaded into SRAM of course, but the
> > > > > whole code until that is done uses so many malloc/free iterations that The
> > > > > simple mall of implementation would require too much memory.
> > > > >
> > > > > And it's the full malloc state variables only that use BSS, not the FAT
> > > > > code.
> > > >
> > > > I've actually faced very similar issues working on our TI AM654x "System
> > > > Firmware Loader" implementation (will post upstream soon), where I need
> > > > to load this firmware and other files from media such as MMC/FAT in a very
> > > > memory-constrained SPL pre-relocation environment *before* I can bring up
> > > > DDR.
> > > >
> > > > Initially, I modified the fat.c driver to re-use memory so it is not as
> > > > wasteful during SYS_MALLOC_SIMPLE. While I'm not proud of this solution [1]
> > > > this allowed us to get going, allowing to load multiple files without
> > > > issues in pre-relocation SPL.
> > >
> > > That seems to point the way to a useful solution I think. We could
> > > have a struct containing allocated pointers which is private to FAT,
> > > and just allocate them the first time.
> >
> > The board_init_f()-based loader solution we use extends beyond MMC/FAT,
> > but also for OSPI, X/Y-Modem, and (later) USB, network, etc.
> >
> > Background:
> > On our "TI K3" devices we need to do a whole bunch of stuff before
> > DDR is up with limited memory, namely loading and installing a firmware
> > that controls the entire SoC called "System Firmware". It is only after
> > this FW is loaded from boot media and successfully started that I can
> > bring up DDR. So all this is done in SPL board_init_f(), which as the
> > last step brings up DDR.
> >
> > Not having BSS available to carry over certain state to the
> > board_init_r() world would lead to a bunch of hacky changes across
> > the board I'm afraid, more below.
> 
> This is really unfortunate.
> 
> It seems to me that we have two choises:
> 
> 1. Hack around with board_init_f() such as to remove the distinction
> between this and board_init_r().
> 
> 2. Enter board_init_r() without DRAM ready, and deal with setting it up there.
> 
> I feel that the second solution is worth exploring. We could have some
> board-specific init in board_init_r(). We already have
> spl_board_init() so perhaps we could have spl_early_board_init()
> called right near the top?
> 
> We can refactor a few of the functions in spl/spl.c so they can be
> called from board-specific code if necessary. We could also add new
> flags to global_data to control the behaviour of the SPL code, and the
> board code could set these.

Let me explore this option. I can probably make something work but I
don't yet see how to do it cleanly, maybe it becomes clearer once I put
some code down. Currently by definition board_init_r() has DDR
available, and much of the code is geared towards it (for example the
calling of spl_relocate_stack_gd() before entering board_init_r() which
will already switch over to DDR).

Also, and not to discourage that we can't improve upon the status quo,
but there is already a ton of boards using such an "early BSS" scheme...

$ git grep --show-function 'memset.*bss' | grep board_init_f | wc -l
35

> > > I wonder if that would be enough for
> > >
> > > >
> > > > In the quest of creating something more upstream-friendly I had then
> > > > switched to using full malloc in pre-relocation SPL so that I didn't
> > > > have to hack the FAT driver, encountering similar issues like you
> > > > brought up and got this working, but ultimately abandoned this
> > > > approach after bundling all files needed to get loaded into a single
> > > > image tree blob which no longer required any of those solutions.
> > > >
> > > > What remained till today however is a need to preserve specific BSS
> > > > state from pre-relocation SPL over to post-relocation SPL environment,
> > > > namely flags set to avoid the (expensive) re-probing of peripheral
> > > > drivers by the SPL loader. For that I introduced a Kconfig option that
> > > > allows skipping the automatic clearing of BSS during relocation [2].
> > > >
> > > > Seeing this very related discussion here got me thinking about how else
> > > > I can carry over this "state" from pre- to post relocation but that's
> > > > probably a discussion to be had once I post my "System Firmware Loader
> > > > Series", probably next week.
> > >
> > > Since this is SPL I don't you mean 'relocation' here. I think you mean
> > > board_init_f() to board_init_r()?
> >
> > Yes that's what I mean. AFAIK relocation in SPL is still called relocation
> > from what I have seen working on U-boot, it just relocates gd and stack
> > but not the actual code (personally I find it misleading calling what SPL
> > does "relocation", but I got used to it).
> >
> > > You can use global_data to store state,
> >
> > I thought the idea was to stay away from gd, so we can eventually get
> > rid of it altogether?
> 
> Not that I know of. It is how we communicate state before we have BSS.

Oh ok I see now, I guess I have taken the comment [1] from
arch/arm/lib/spl.c out of context.

[1] https://github.com/u-boot/u-boot/blob/master/arch/arm/lib/spl.c#L22

> It is how we communicate state before we have BSS.

Understood.

> > > or malloc() to allocate memory and put things there.
> >
> > The challenge with potentially having to incorporate such a custom
> > solution for state preservation into several drivers as explained
> > earlier (SPI, USB, network, etc.) is that it does not appear to scale
> > well. I think using BSS instead would make all those additions cleaner
> > and simpler.
> >
> > > But using BSS seems wrong to me.
> >
> > I've seen you saying this a few times :)
> > Why?
> 
> Driver model does its own allocate of memory and this is all attached
> to the DM structures.
> 
> Drivers themselves cannot assume they are the only instance running,
> so data should be attached to their private-data pointers. Similarly
> for uclasses, if we put everything in the uclass-private data, then we
> don't need BSS and don't have any problems dealing with whether it is
> available yet. In general, BSS creates a lot of problems early in
> U-Boot's execution, and we don't actually need to use it.

Yes, understood. The DM concept with private-data pointers is quite
clean from an OOP point of view.

> If you look at the DM design you'll see that we try to avoid malloc()
> and BSS as much as possible. I suppose this series is another example
> of why :-)
> 
> >
> > > If you are doing something in board_init_f() in SPL that needs BSS,
> > > can you not just move that code to board_init_r()?
> >
> > I need to access media drivers in board_init_f(), for which currently
> > I'm using BSS so I can preserve some limited state, such as that the
> > peripheral init was done, so that it doesn't get re-done by the actual
> > SPL loader later. board_init_r() requires DDR to be available which I
> > can't use without doing all that work in board_init_f() first to
> > load/start the system controller firmware, so it's a bit of a chicken
> > and egg issue here.
> 
> Let's try moving the egg into board_init_r()  and putting the chicken
> after it, as mentioned above.

Well I'll give this a shot.

> >
> > My system firmware loader patch series is about ready and I was planning
> > on posting it tomorrow. How about with the entire approach being in the
> > open we use this as an opportunity to re-look at potential alternative
> > solutions...
> 
> Sure. I hope I've explained my POV above.

Yes thanks for the review and comments.

--
Andreas Dannenberg
Texas Instruments Inc